Early data suggests that the current 2014-2015 flu season could be severe. The CDC urges immediate vaccination for anyone still unvaccinated this season and recommends prompt treatment with antiviral drugs for people at high risk of complications who develop flu.
So far this year, seasonal influenza A H3N2 viruses have been most common. There often are more severe flu illnesses, hospitalizations, and deaths during seasons when these viruses predominate. For example, H3N2 viruses were predominant during the 2012-2013, 2007-2008, and 2003-2004 seasons, the three seasons with the highest mortality levels in the past decade. All were characterized as “moderately severe.”
Increasing the risk of a severe flu season is the finding that roughly half of the H3N2 viruses analyzed are drift variants: viruses with antigenic or genetic changes that make them different from that season’s vaccine virus. This means the vaccine’s ability to protect against those viruses may be reduced, although vaccinated people may have a milder illness if they do become infected. During the 2007-2008 flu season, the predominant H3N2 virus was a drift variant yet the vaccine had an overall efficacy of 37 percent and 42 percent against H3N2 viruses.
“It’s too early to say for sure that this will be a severe flu season, but Americans should be prepared,” said CDC director Tom Frieden, M.D., M.P.H. “We can save lives with a three-pronged effort to fight the flu: vaccination, prompt treatment for people at high risk of complications, and preventive health measures, such as staying home when you’re sick, to reduce flu spread.”
Depending on the formulation, flu vaccines protect against three or four different flu viruses. Even during a season when the vaccine is only partially protective against one flu virus, it can protect against the others.
“While the vaccine’s ability to protect against drifted H3N2 viruses this season may be reduced, we are still strongly recommending vaccination,” said Joseph Bresee, M.D., Chief of the Influenza Epidemiology and Prevention Branch at the CDC. “Vaccination has been found to provide some protection against drifted viruses in past seasons. Also, vaccination will offer protection against other flu viruses that may become more common later in the season.”
Influenza viruses are constantly changing. The drifted H3N2 viruses were first detected in late March 2014, after World Health Organization (WHO) recommendations for the 2014-2015 Northern Hemisphere vaccine had been made in mid-February. At that time, a very small number of these viruses had been found among the thousands of specimens that had been collected and tested.
A committee of experts must pick which viruses to include in the vaccine many months in advance in order for vaccine to be produced and delivered in time for the upcoming flu season. There is always the possibility that viruses will drift during that time.
Influenza activity is currently low in the United States as a whole, but is increasing in parts of the country. “We are just at the beginning of the season. It’s not too late to get your vaccine,” Dr. Frieden says. “There are four different strains of flu circulating: The B strain, the h1 strain, the well matched H3 strain, and the poorly matched H3 strain, and only time will tell which of them, if any, will predominate for the following weeks and months of this year's flu season. Flu always has a potential to be serious, but H3N2 viruses tend to be associated with more severe seasons. The rate of hospitalization and death can be twice as high as or more than in flu season when H3 doesn't predominate.”
Influenza antiviral drugs – Tamiflu (oseltamivir) and Relenza (zanamivir) can reduce severe complications such as hospitalization and potentially death for people who are at high risk of serious flu complications or are very sick. It is especially important to get antiviral medicines quickly if you have flu. They work best when you start them within two days of the beginning of flu symptoms, and the CDC strongly recommends that if doctors suspect the flu in someone who may be severely ill from the flu, they do not wait for the results of a flu test before starting antivirals.
Those at high risk from influenza include children younger than five years old (especially those younger than two years old); adults 65 years and older; pregnant women; and people with certain chronic health conditions such as asthma, diabetes, heart or lung disease, and kidney disease.
The CDC recommends that people at high risk check with their doctor or other health care professional promptly if they get flu symptoms. Flu symptoms can include fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills, and fatigue.
See the CDC Announcement
See the CDC Telebriefing Transcript
See also Medical Law Perspectives, January 2013 Report: Vaccines: An Ounce of Prevention May Lead to a Pound of Injury